Issue 50

Z.-y. Han et alii, Frattura ed Integrità Strutturale, 50 (2019) 21-28; DOI: 10.3221/IGF-ESIS.50.03 21 serious environmental pollution and huge economic losses. Therefore, stress corrosion failure has aroused a worthy concern for safe service of the submarine pipeline [5-6]. Stress corrosion cracking (SCC) is a low-stress brittle cracking phenomenon of pipeline steel under the coupling of stress and a specific corrosive environment. Unlike corrosion fatigue, stress corrosion has an obvious dependence on material and environment. SCC occurs in an environment that contains corrosive media such as CO 2 , H 2 S, Cl - , OH - , HCO 3 - , and CO 3 2- [7]. According to the pH value of the service environment, SCC of the pipelines steel can be divided into high, near neutral and low pH types. At present, the research on the SCC of the pipeline steel mainly focuses on the near neutral and high-pH SCC. The high-pH SCC mainly occurs in the soil environment (pH 8~10.5) with high concentration of Na 2 CO 3 and NaHCO 3 and induces an intergranular cracking [8]. The mechanism of the high-pH SCC is consistently recognized to be the film rupture and slip dissolution [9-10]. However, the transgranular SCC often occurs in the near neutral (pH 6~8) environment. At present, the stress corrosion mechanism of pipeline steel in a near neutral environment has been widely accepted as the interaction of anodic dissolution and hydrogen embrittlement. However, in a marine environment, the situation may be more serious. For example, high concentration of Cl - accelerates the pitting corrosion and greatly promote the crack nucleation of SCC [11]. Additionally, the subsea is a complex and changeable corrosive environment with various hydrostatic pressures, varying DO contents and pH values at different depths [12]. A number of studies have been performed to investigate the corrosion behavior of pipeline steel in the natural marine environment [13-15]. However, research work on the stress corrosion behavior of X80 pipeline steel in the natural seawater is very limited. In this paper, the slow strain rate tensile (SSRT) tests of X80 steel were conducted to study the sensitivity of stress corrosion cracking in the natural seawater with different DO contents. SEM analyses of the lateral and fracture surface were complemented to investigate the mechanism of stress corrosion in different environments. Electrochemical polarization and impedance spectroscopy of the stressed X80 samples in the seawater were measured to explain the effect of DO on the corrosion and stress corrosion behavior. Also, a model that stress concentration promotes the anodic dissolution and the SCC fracture is adopted to explain this accelerating effect once the corrosion pits and microcracks are formed. Figure 1 : Microstructure of X80 steel. E XPERIMENTAL PROCESS Samples he selected X80 pipeline steel for stress corrosion SSRT tests contains (w.t. %) 0.055 C, 1.392 Mn, 0.328 Mo, 0.202 Si, 0.264 Ni, 0.0017 P, 0.002 S, 0.032 Cr, 0.017 Al, and Fe rem. The microstructure of X80 pipeline steel is shown in Fig. 1. The microstructure is composed of acicular ferrite (AF), polygonal ferrite (PF), bainite (B), and many small precipitates dispersed in the matrix. The tested yield strength σ s and tensile strength σ b of X80 steel are 680MPa and 710MPa, respectively, which indicated that the X80 steel has a better ability to resist deformation. The plate-shaped specimen shown in Fig. 2 is machined from an X80 steel plate by wire-cutting and fine grinding to achieve the requirement of accuracy. According to the literature [11], the DO content in seawater of China Sea varies from 2.6 to 6.8mg/L, gradually decreasing from 6.8% at shallow sea to 2.6 % along with the depth of the seawater. To analyze the effect of DO on the stress corrosion AF B PF T